Yes, I've crashed a lot of two-stagers. Some of them have been interesting though. I've posted many of my staged flights here:
https://www.youtube.com/results?search_query=jiminaus50
Maybe since this is a kit thread, I'll describe what you'd get if you got a "Jim" kit. I don't make kits, but if I did, this is what it would be. My kit might not suit you at all - it's just another way to do it.
The kit would be 3" to 3". It would be fiberglass and designed to fly anywhere from 3K to 25K. The pics below are from an existing rocket I built maybe 12 years ago? That rocket has probably flown a couple of dozen times, and there are a few concepts from it that I would keep.
(Temporary rant on) My kit would have surface mounted fins. I think many fiberglass kits come with through the wall because they can be built by folks with a wider range of skill levels. But I have never understood why you would want to take a perfectly good fiberglass tube and cut slots in it. In 3" with a permanent motor mount, surface mounted fins with reasonable reinforcement will be stronger and lighter than through the wall. (Temporary rant off) My fins would be 5/32 or 3/16", and I would include just enough carbon for a couple layers of tip to tip. This would stay inside the perimeter of the fins - i.e., the carbon would stop short of the edges of the fins, and then I would use epoxy/West 410 or the like to smooth everything out.
The booster would be very simple. I'd go with a 54" motor mount with a retainer and just use motor deploy, or a chute release. A chute release is a little risky because apogee deployment might not be very accurate for a two stage booster, which increases the risk of the chute release releasing the chute early upon deployment. I calculate the delay time by performing a simulation but without separating the rocket or lighting the sustainer. I calculate the delay time in that configuration and then take 2/3 of that value for setting the motor delay. I get close most of the time. I'd design the booster with a piston as I think they greatly increase deployment reliability, so the booster would just have an open top.
The interstage coupler would be the "nose cone" for the booster and it would slide up into the bottom of the sustainer. A caliber or just a little bit more into the tubes on either side. A small switch band goes on the outside and a bulkhead goes at the appropriate location on the inside (leaving enough room for a separation charge below the sustainer motor. A good fiberglass tube would be fine for the interstage coupler in the kit, but I'd add a few wraps of carbon to the inside of the tube before adding the bulkhead. Two stagers tend to fail at this point, so stronger is better.
I suppose some electronics could go into the bottom side of the interstage coupler. This might allow better apogee detection and the separation charge could be done from that point. I don't light motors from the interstage coupler though (that voids the kit warranty). Be sure to shear pin the interstage coupler to the top of the booster, just like a nose cone. Also, if electronic apogee deployment is used, then the booster could be constructed minimum diameter.
My sustainer would be pretty conventional. Again, 54mm mount with retainer. The bottom ring of the mount could fit against the top of the interstage coupler. I'd use three rings on the motor mount and I would also mount the fins a little higher on the airframe (around those rings). The secret to sound surface mounted fins is to prevent the underlying tube from deforming.
The sustainer would have a zipperless coupler design. The wires for the motor ignition and the separation charge would pass through a brass conduit from the top of the zipperless coupler to the bottom motor mount centering ring. I connect the wires to wires from a conventional altimeter bay just using a simple twisted breakwire.
The first pic shows the altimeter bay in the upper air frame with the wires passing through the drogue section. I've done it this way for many dozens of flights and have never had a problem with the wires inhibiting drogue deployment in some manner. The wires need to be attached securely within the bay so that the electronics don't get yanked out upon separation.
The pic also shows an upper airframe that is in two parts (with the bay going between them. The kit would only have a single tube, and the bay would slide in from the top, sit on a ring and get pinned with rivets or the like. Air frame breaks should be avoided on two stagers wherever possible.
For electronics, I have always used either an Easymega or a Raven for staging. Both altimeters have some quirks and need to be fully tested before use. But, they both allow an altitude check, which is essential for staging. There are other altimeters that do this also, including the RRC3+ and one of the Egg-things and probably the Marsa. You would use the Easymega or Raven with another dual deploy altimeter, or perhaps use a pair of RRC3+ altimeters. Back when I started staging, I used an ARTS II, which could fire a channel at a pre-set altitude on the way up, combined with a timer that actually lit the motor. The output of the ARTS II was connected to a transistor switch. If the altitude was reached before the timer fired, it would open the transistor switch and allow the timer current to the igniter. It worked well for several years. It is critical to use an altitude check with any staged rocket. Tilt is nice, but secondary I think.
The pathway to the igniter would include two switches. One would open the igniter leads and the other would short them out. The one opening the circuit would be on the altimeter side of the shunt, which is a lot like twisting the leads of the igniter together. The design of these circuits is a topic for another day.
Staging delays are another subject too. On a slower flight, a few seconds is usually OK. If the boost will approach mach, then you can let the sustainer slow down to perhaps 500 ft/sec with no problems. If above Mach, then there is a choice. I have generally let my rockets fall through mach to perhaps 700 ft/sec to reduce the top speed of the sustainer. But there are reasons to consider lighting above mach such as trying to maintain a straighter trajectory.
So, above the bay is another piston and then the cone of your choice.
Anyway, that's my kit. Thanks for letting me design it on your thread!
Jim